A known electromagnetic clutch includes a rotor, which is rotated by a rotational drive force outputted from a drive source, and an armature, which is rotated when the armature is attracted to and is coupled with the rotor by a magnetic force.
In order to improve a magnetic efficiency, each of the rotor and the armature is formed such that a plurality of circular plates, which are made of a magnetic material, is concentrically placed about a rotational axis, and an annular ring, which is made of a non-magnetic material, is placed between adjacent two of the circular plates (see, for example, Patent Literature 1).
More specifically, a ring insertion groove is formed in an inner peripheral surface or an outer peripheral surface of each corresponding plate, and the ring is inserted between the adjacent plates. Thereafter, the ring is pressed with a die to plastically flow the ring into the ring insertion groove, so that the adjacent plates and the ring are joined with each other.
However, in the above-described prior art electromagnetic clutch, it is required to have a predetermined remaining straining force in the non-magnetic ring, which has been plastically flowed, in order to satisfy a predetermined mechanical strength with respect to shearing, compressing, bending or the like. Therefore, the ring insertion groove needs to have a complex shape, so that the processing of the ring insertion groove is not easy.
Furthermore, in order to plastically flow the ring, the material of the ring has a smaller deformation resistance in comparison to a material of the plates. That is, the ring can be easily deformed. Therefore, the ring may be easily deformed by a load, which is applied to the ring and/or the plates by a belt in a direction perpendicular to a rotational axis, and thereby a crack may be disadvantageously formed in the ring.